A problem faces the designer of a muffler for a motor vehicle, where the muffler had to, of necessity, be located behind the rear pair of wheels (or behind the rear axle) with a consequentially short tailpipe. In order to silence high frequencies other designers invariably specified a large muffler volume, with a large acoustic compliance, and with internal devices such as a perforated main flow conduit which have a low acoustical inertance at low frequencies and a high inertance at high frequency. Such an arrangement is satisfactory at high frequencies (over approximately 1000 Hz), however, the compliance of the muffler volume is in resonance with the inertance of the gas in the short tailpipe at a particular low frequency, usually in the range 30 to 100 Hz with typical dimensions in practical use. There is thus a frequency range, centred about this resonance for which the insertion loss becomes very poor and the radiated exhaust noise becomes high.
An obvious solution to the problem is to increase the tailpipe length considerably to increase the tailpipe inertance and thereby place the problem resonance well below the operating range of frequencies. This was the approach of Ishida in Australian Patent application No. 48297/79. In that specification the tailpipe was lengthened by taking it out of the front head plate of the muffler and, with a nearly 180 degree bend, taking it to the rear of the vehicle, thereby lengthening the tailpipe considerably but at considerable expense. This arrangement has been regarded by some as being clumsy and expensive, and in some applications, as being very inconvenient.
Another difficulty encountered is the provision of a muffler to be effective at frequencies ranging from above the low frequency (suppressed) resonance to those for which the tailpipe itself is of the other of half a wavelength long.
By incorporating a high frequency side-branch Helmholtz resonator, as a muffler upstream of a short tailpipe, exhaust noise at frequencies near the Helmholtz resonance frequency of very approximately 1000 Hz is greatly attenuated, whereas low frequency sound near 50 Hz is poorly attenuated or even intensified. This greater low frequency sound is related to the resonance of the Helmholtz resonator volume's compliance and the tailpipe gas's inertance, notwithstanding the fact that the Helmholtz resonance frequency of the muffler as a side-branch is high. By incorporating an additional side-branch Helmholtz resonator within the same muffler housing with a Helmholtz resonance frequency near to that of the low frequency intensified sound, not only does a great attenuation of the low frequency sound result, but attenuation of sound now results over the entire relevant audible range.